Ca. Wijker et al., MUTATION SPECTRUM IN THE LACI GENE, INDUCED BY GAMMA-RADIATION IN AQUEOUS-SOLUTION UNDER OXIC CONDITIONS, Mutation research. Fundamental and molecular mechanisms of mutagenesis, 403(1-2), 1998, pp. 137-147
Irradiation of DNA in a cellular environment leads to many types of DN
A damage, resulting from various effects of gamma-radiation. One of th
ese effects is the formation of water-derived radicals (e.g., OH radic
als), which are formed in the vicinity of DNA (indirect effect). To st
udy the influence of the indirect effect on gamma-radiation-induced mu
tations, a newly constructed plasmid, containing the lad gene as a tar
get gene, was irradiated with Co-60 gamma-radiation in aqueous solutio
n, in the presence of oxygen. Under these circumstances, only OH radic
als will be responsible for the induced mutations. Sequence analysis o
f the gamma-radiation-induced mutations showed that 96% of all mutatio
ns were base pair substitutions, 87% of which occurred in the lad gene
, the others are formed in the lac operator part. All gamma-radiation-
induced mutations in the lad gene occurred exclusively on G:C base pai
rs, and no mutations at A:T base pairs could be detected. In the spont
aneous mutation spectrum, 83% of all mutations were base pair substitu
tions, 35% of which occurred in the lacI gene and 48% in the lac opera
tor part. Base pair substitutions on G:C base pairs were very similar
in the gamma-radiation-induced and in the spontaneous mutation spectru
m, implying a high contribution of OH radicals to spontaneous mutagene
sis. A:T to G:C transitions accounted for 10% of all spontaneous base
pair substitutions in the lad gene and are probably the result of effe
cts, other than just OH radicals. It can be concluded that OH radicals
are an important source for mutations at G:C base pairs. In this pape
r, the extracellular gamma-radiation-induced mutation spectrum is also
compared to the previously obtained, intracellular gamma-radiation-in
duced mutation spectrum of the lad gene. Comparison shows some differe
nces, such as relative high amounts of mutations at A:T base pairs, G:
C to T:A transversions and frameshift mutations in the intracellular g
amma-radiation-induced mutation spectrum, as compared to the extracell
ular gamma-radiation-induced mutation spectrum. Since the extracellula
r gamma-radiation-induced mutation spectrum shows that OH radicals are
mainly responsible for base pair substitutions on G:C base pairs, mut
ations at A:T base pairs in the intracellular gamma-radiation-induced
mutation spectrum are apparently the result of additional or other fac
tors. (C) 1998 Elsevier Science B.V. All rights reserved.